The present invention relates to a method and apparatus for producing high integrity transverse closure heat seals in tubular thermoplastic bag stock and is particularly applicable to making such closure seals in bag stock having transverse cross-sectional properties which make difficult the application of such transverse closure seals without melting, singeing or otherwise degrading the bag stock.
The invention finds its primary use in the fabrication of flexible, thermoplastic, vapor-sterilizable bags for storing articles in bacteria-free condition such as those shown in U.S. Pat. Nos. 3,472,369 and 3,761,013 issued to the present inventor on Oct. 14, 1969 and Sept. 25, 1973, respectively.
Such containers are formed from a continuous sheet of polyethylene, polypropylene, or the like, the side margins of which are folded inwardly along longitudinal fold lines. The margins, which typically are in overlapping relationship, define a longitudinal access opening. A closure membrane or tear strip of paper of "Tyvek" (a spun polyolefin of the duPont Company), for example, covers the access opening and is bonded to the folded sheet by a longitudinally-extending heat seal on each side of the opening. The ends of the bag are closed by transverse heat seals joining the various plies of plastic sheet and the tear strip.
The transverse closure heat seals must be of high integrity, that is, they must be completely impervious to the passage of bacteria. Such transverse heat seals are generally applied by a single heat seal bar brought into contact with the front or upper face of the bag stock, that is, the face to which the tear strip is attached, as the bag stock passes about a feed drum having a resilient outer surface which functions as a back-up surface for the transverse heat seal bar.
It will be evident that the above-described bags, viewed in transverse cross-section, have thicknesses and material compositions that vary along the transverse section, ranging from two plies of plastic sheet along the outer margins of the bag to four plies, consisting of three plies of plastic and the single ply tear strip, in the central portion. Such differential thicknesses and composition make the application of a transverse heat seal of the required high integrity difficult. If the transverse seal bar is heated to a temperature sufficient to assure adequate sealing of the central, thicker portion of the bag within a reasonably short dwell period, singeing, melting, burnthroughs or other degradation of the bag stock material adjacent the heat seal bar frequently occurs. This is especially a problem with "Tyvek" which deteriorates rapidly when exposed to excessive temperatures.
The foregoing problems are also particularly evident in the fabrication of gusseted bags, that is, bags provided with reentrant folds, and heavy-walled bags with sheet thicknesses ranging, for example, from 6 to 8 mils.
In the foregoing instances, the transverse heat seal bar temperatures of 450.degree.-600.degree. F. typically in use today result in excessive numbers of rejected bags. As another consequence, the anti-stick coverings such as "Teflon" (trademark of the duPont Company for plastic consisting of a tetrafluoroethylene polymer) over the heat seal bars tend to rapidly deteriorate at these temperatures and thus must be replaced quite frequently.
Attempts have been made to overcome the aforementioned problems by lowering the temperature of the transverse heat seal bar. However, this requires a concomitant increase in the dwell period and for this reason has not been successful in preventing the formation of seals of inadequate integrity. Production rates are also decreased, of course, as a result of the increase in dwell.
Another approach to solving the foregoing problems has been to maintain the surface of the feed drum at an elevated temperature to decrease the temperature gradient across the thickness of the bag stock so that the heat seal bar can be maintained at a substantially lower temperature. However, it has been found that the bag stock not only has a tendency to stick to the heated drum surface but coherence of the entire front and back faces of the bag stock takes place as well.
The application of the transverse closure heat seal by a pair of opposed heat seal bars operated to simultaneously clamp the bag stock between them has also been attempted. However, even when heated to only moderate temperatures such seal bars tend to excessively soften the plastic and special tension control and cooling apparatus is required to prevent the bag stock from being pulled apart at the seal. Moreover, with this technique the alignment and temperature of the sealing bars is critical and because a resilient back-up surface cannot be used, the uniformity of the gauge, composition and other physical properties of the bag stock must be carefully controlled.